The present invention relates to induction processing with the aid of a conductive shield. More particularly, it relates to the use of a conductive shield to prevent overheating of parts of a workpiece during magnetic induction heating.
The physical properties of metal workpieces, such as those made from iron and iron alloys, can be improved by the process of heat treatment. During heat treatment, a workpiece is heated above its solution temperature and then quenched to reduce the temperature. Heat treatment hardens the surface of the workpiece, leading to improved surface durability and performance.
In iron alloys the solution temperature of interest is the well known austenitizing temperature at which the austenite phase transition occurs. It is important that the workpiece not be further heated so much as to exceed its fusion temperature or its melt temperature. If a workpiece is heated above the austenitizing or solution temperature as high as its fusion or melt temperature, generally the material performance will be adversely affected, as the portion of the workpiece heated to the fusion temperature or the melt temperature will be degraded or weakened.
The heat-hardened portion of a workpiece is more brittle than the untreated portion. For this reason it is often desirable to apply heat treatment so as to harden only the surface, or only a portion of the surface, of a workpiece. The part of the workpiece that is not heat treated will remain ductile while the heat treated area will become hardened. For example, a surface may be hardened while leaving the center of a workpiece relatively more ductile.
One method of heat treating involves heating the workpiece in an oven. However, the heat treatment in an oven is relatively slow and expensive, and tends to heat the workpiece through, rather than to heat only the surface. Surface heat treatment can be conveniently accomplished by induction heating. In induction heating, the workpiece is exposed to a time varying magnetic field. The magnetic field induces currents in the surface of the workpiece which causes the surface to increase in temperature. However, a drawback of induction heating is that it is subject to edge effects where areas of the workpiece which are induced current concentrators heat up faster in a magnetic field than other areas. Such uneven heating can result in overheating of certain parts of the workpiece during induction heating. Should such overheating result in the raising of a portion of the workpiece to above the fusion temperature or the melt temperature, it can lead to a weaker part with significantly shortened life.
Workpieces such as gears are often used in very corrosion prone environments, yet are expected to obtain a long service life such as up to thirty years or more. An example of such application is found in gear boxes used in the control systems of commercial aircraft. Such gears can be cadmium plated, placed in very expensive sealed units, and kept oiled in order to obtain the expected life. It would be desirable to use corrosion resistant steel or stainless steel in such applications so as to achieve the long life without the added expense of the cadmium plating. However, the traditional carburizing process used for hardening the gears destroys the corrosion resistance of stainless steel. Furthermore an alternative induction hardening process has a tendency to overheat the root area of the gear because of the relationship between the Curie temperature, the austenitizing or solution temperature, and the fusion or melt temperature.
All ferrous materials are characterized by a Curie temperature. Below the Curie temperature, the magnetic permeability of the material is greater than 1, while at the Curie temperature and above, the material loses its magnetic properties and the magnetic permeability becomes equal to 1. When the magnetic permeability is greater than 1, the workpiece has a large interaction with a magnetic field, so that induction heating is very rapid in a part of the workpiece that is below the Curie temperature. As the workpiece is heated during the induction heating process, parts of the workpiece approach the Curie temperature. At the Curie temperature, the magnetic permeability goes to 1 so that part of the workpiece heats up at a slower rate. Meanwhile other parts of the workpiece that have not yet reached the Curie temperature are being heated at an increased rate by the magnetic field. When those parts of the workpiece reach the Curie temperature, they will also start to then heat up at a slower rate. The result is that during induction heating, a workpiece is heated relatively quickly up to the Curie temperature. Above the Curie temperature, the induction heating process heats the workpiece up at a slower rate. However, because of the induced current concentrating effects of some of the surfaces of the workpiece, the heating is not uniform. Some parts of the workpiece are heated at a higher rate than other parts when the workpiece is above the Curie temperature.
During heat treatment it is desirable to heat the surface of interest, such as the teeth of a gear, to above the austenitizing or solution temperature of the material so as to provide a hardened surface for prolonged tool life. At the same time, it is desirable not to heat any of the other parts of the workpiece above its fusion temperature or its melt temperature. As discussed above, such would result in weaker parts with shortened life. However, because certain surfaces of the workpiece tend to concentrate the induced current, those surfaces are heated at a higher rate. It is therefore possible that by the time a gear tooth reaches the temperature required for heat hardening, other parts of the workpiece such as the roots of the gear will have been heated above the fusion temperature. This is particularly the case in corrosion resistant steel or stainless steel where the fusion temperature is not much higher than the austenitizing temperature. For example, one of the alloys has a Curie temperature of 1376xc2x0 F. but has to be heated to the austenitizing temperature of 1965xc2x0 F. for heat treatment. The fusion temperature is only 2200xc2x0 F. Thus there is a relatively small temperature window between the austenitizing temperature and the fusion temperature. The problem is less severe with conventional gear steels. For example, one such alloy has a Curie temperature of 1444xc2x0 F. and an austenitizing temperature of 1700xc2x0 F. Furthermore, there is no fusion temperature and the melting temperature is 2700xc2x0 F. It can be seen that in the conventional gear steel there is a temperature window of about 1000xc2x0 between the austenitizing temperature and the melting temperature. Therefore in conventional gear steels the problem of overheating the roots of a gear during an induction hardening process are not as severe as when corrosion resistant steels or stainless steel are used.
It would be desirable to provide a method for induction heating of gears and other workpieces made of materials such as corrosion resistant steel or stainless steel. Desirably, such a method would avoid the drawback of overheating of surfaces of the workpiece that concentrate the induced current during induction heating. By such a method, the parts of the workpiece to be heat hardened should be heat treated to a temperature above the austenitizing temperature, while other parts of the workpiece do not exceed the fusion temperature or the melting temperature during the process.
A method is provided for induction heating a workpiece made of a metal or metal alloy, especially iron or an iron alloy. The workpiece has at least one induced current concentrating surface such as a hole through a shaft, an edge, or a gear root. The method is carried out by placing a non-magnetic conductive shield in proximity to a current concentrating surface of the workpiece. The workpiece with the shield in place is then exposed to a time varying magnetic field. The magnetic field has a frequency sufficient to cause eddy currents in the surface of the workpiece, and the conductive shield is placed in sufficient proximity to reduce or eliminate the eddy currents in the portion of the workpiece covered by the shield. Exposure to the magnetic field is carried out until at least a part of the unshielded portion of the workpiece is heated above the austenitizing or solution temperature of the material. At the same time, the shielded portion is protected from overheating by the shield so that it obtains a temperature below the fusion temperature or the melt temperature of the material. The workpiece is then quenched to complete the hardening process.